The hepatic activity of betaine-homocysteine methyltransferase is a complex function of the content of methionine in the diet. Enzyme levels are lower in the livers of rats fed a 0.3% methionine diet than in livers of animals maintained on either methionine-free or excessive methionine (1.0%) rations. Because activities are increased at both extremes of the spectrum of dietary methionine intake, the betaine-homocysteine methyltransferase reaction may function both to maintian tissue concentrations of methionine when intake of this amino acid is limited and to remove homocysteine when methionine intake is excessive.